Purpose: A series of melanocortin-4 receptor (MC4R) agonists, developed for use as anti-obesity agents, were found to have unusual pharmacokinetic behavior arising from excessive retention in the liver, with nearly undetectable levels in plasma following oral administration in mice. This work investigates the molecular basis of the prolonged liver retention that provided a rational basis for the design of an analog with improved behavior.
Materials and methods: The livers of mice were harvested and techniques were utilized to fractionate them into pools differentially enriched in organelles. The distribution of organelles in the fractions was determined using organelle-specific enzymatic assays. Livers from mice dosed with drug were fractionated and comparisons with organelle distributions assisted in determining the subcellular localization of the drug. Further analysis in cell culture systems was used to confirm results from liver fractionation studies and also allowed for more extensive evaluations to examine the mechanism for organelle compartmentalization
Results: Fractionation of livers following oral administration of the agonist showed sequestration in lysosomes. Subsequent evaluations in a cell culture system confirmed this finding. Agents used to disrupt acidification of lysosomes led to decreased lysosomal accumulation of the drug, which implicated a pH-partitioning type sequestration mechanism. These findings led to the rational synthesis of an analog of the parent compound with properties that reduced lysosomal sequestration. When this compound was examined in mice, the liver retention was found to be greatly reduced and plasma levels were significantly elevated relative to the parent compound.
Conclusions: Weakly basic drugs with optimal physicochemical properties can be extensively sequestered into lysosomes according to a pH-partitioning type mechanism. When administered orally in animals, this particular sequestration event can manifest itself in long term retention in the liver and negligible levels in blood. This work revealed the mechanism for liver retention and provided a rational platform for the design of a new analog with decreased liver accumulation and better opportunity for pharmacokinetic analysis and therapeutic activity.